15-441: Computer Networks Recitation 11/15 Agenda 1. Quick Project - - PowerPoint PPT Presentation

15-441: Computer Networks

Recitation 11/15

Agenda

• 1. Quick Project 3 Reminder
• 2. Background on Bitrate Adaptation
• 3. Background on CDN & DNS Redirect

Project 3: Quick reminder

Grade Deadline

60% Nov 22

CP1 Start early! Do not wait until the last day! CP2 Grade Deadline

40% Dec 6

Agenda

• 1. Quick Project 3 Reminder
• 2. Background on Bitrate Adaptation
• 3. Background on CDN & DNS Redirect

Internet Video Data-plane

Video Source Encoders & Video Servers CMS and Hosting Content Delivery Networks (CDN) ISP & Home Net Screen Video Player

Terminology

▪ Bitrate

• Information stored/transmitted per unit time
• Usually measured in kbps to mbps
• Ranges from 200Kbps to 30 Mbps

Adaptive Bit Rate with HTTP Streaming

▪ Encode video at different levels of quality/bandwidth ▪ Client can adapt by requesting different sized chunks ▪ I.e., if downloading a chunk takes too much time, choose a lower

bit rate for the next chunk

▪ Chunks of different bit rates must be synchronized

• All encodings have the same chunk boundaries and all chunks start

with key frames, so you can make smooth splices to chunks of higher or lower bit rates

Bit Rate Selection

• Each chunk represents a certain play time
• Transfer time of chunk must be shorter than the play time
• Learn from previous chunk transfers what the available bandwidth is
• n network path from server to client
• Use this to estimate predicted transfer time (PTT) of future chunks
• General approach to adapting bit rate:
• Decrease bit rate if PTT is close to/higher than play time
• Increase bit rate if PTT is significantly lower than play time
• Many variants: what thresholds, hysteresis, etc.

Bit Rate Selection - Implementation

• Manifest file lists list multiple URLs for

each chunk, one for each different bit rates

• Client estimates PTT for the chunk

based on previous transfer times

• Selects best bit rate
• PTT is below threshold
• QoE considerations
• Buffer status, …

Server Bandwidth Estimation GET Chunk Bit Rate & Chunk Selection TT Manifest File Client

Agenda

• 1. Quick Project 3 Reminder
• 2. Background on Bitrate Adaptation
• 3. Background on CDN & DNS Redirect

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Content Distribution Networks (CDNs)

• The content providers are the CDN

customers. Content replication

• CDN company installs hundreds of

CDN servers throughout Internet

• Close to users
• CDN replicates its customers’

content in CDN servers. When provider updates content, CDN updates servers

• rigin server

in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia

What is the CDN?

• Edge Caches: work with ISP and networks everywhere to install edge

caches

• Edge = close to customers
• Content delivery: getting content to the edge caches
• Content can be objects, video, or entire web sites
• Mapping: find the “closest” edge server for each user and deliver content

from that server

• Network proximity not the same as geographic proximity
• Focus is on performance as observed by user (quality)

Server Selection

• Which server?

○ Lowest load: to balance load on servers ○ Best performance:

to improve client performance

○ Based on Geography? RTT? Throughput? Load? ○ Any alive node: to provide fault tolerance

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Server Selection

• How to direct clients to a particular server?

○ As part of naming: DNS redirect ○ As part of application: HTTP redirect ○ As part of routing: anycast, cluster load balancing

• To be noticed, in DNS redirect, DNS server needs to know

the IP address, and also needs to be able to map that into a closeness measure for each server cluster

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Finding the “Closest Edge Cache – Example: Akamai DNS Redirect